1. Field of the Invention
This invention relates to a process for electrolytic production of chlorine dioxide used in bleaching of paper pulp and disinfection or deodorization of potable water. More particularly, this invention relates to a process for producing chlorine dioxide by electrolyzing a solution of alkali metal chlorite supplied to an anode compartment in a diaphragm electrolytic cell.
2. Description of the Prior Art
Chlorine dioxide is produced by a chemical method wherein chlorate or chlorite is used as a starting material or by an electrochemical method wherein chlorine dioxide is generated by electrolysis of chlorite. The method which obtains chlorine dioxide from chlorate uses a reducing agent that acts on the chlorate in a strong acid solution and the method has various modifications according to the type of reducing agent. A typical example uses sodium chloride as the reducing agent (e.g. U.S. Pat. No. 3,563,702). Examples of the chemical method include decomposition of chlorite with acid (e.g. Swiss Patent No. 527,126) and reaction between chlorite and chlorine (e.g. French Patent No. 2,086,624). One example of the electrochemical method is diaphragm electrolysis wherein a chlorite solution is used as anolyte and caustic alkali or sodium chloride solution is used as catholyte (e.g. U.S. Pat. No. 3,763,006). In any of these methods, chlorine dioxide whose concentration is more than 15 vol% is explosive and hence is usually recovered after being diluted with an inert gas such as air.
The chemical method, particularly the one that uses chlorate is primarily used to produce chlorine dioxide for pulp bleaching and is in most cases operated on a relatively large scale. Liquid chlorine is conventionally used to disinfect potable water, but it has a tendency to form trihal methane (THM and primarily chloroform) being a suspected carcinogen in water, so researchers are getting interested in the use of chlorine dioxide instead of liquid chlorine (Journal of the Society of Waterworks, No. 546, March 1980, pp. 123-124). For use in water treatment plants, a small system for producing chlorine dioxide is a must. For example, a chemical system that is designed to generate less than about 100 kg of chlorine dioxide per day involves a side reaction and with this system, it is difficult to control the complex mechanism of the reaction of the formation of chlorine dioxide, and hence a lot of equipment cost and labor is required to achieve the proper control of chlorine dioxide produced. What is more, such system is not completely safe. On the other hand, the electrochemical method is adaptive to a small-scale apparatus, involves less side reaction and produces pure chlorine dioxide, is capable of controlling the production of chlorine dioxide simply by controlling electrolytic current according to a change in the quality of water supplied to the water treatment plant or a change in the demand for water at different time during the day. Chlorine dioxide may explode at high concentrations, but unlike the chemical method the electrochemical method is very safe because when air blowing is suspended by power failure or some other reasons, the production of chlorine dioxide is also stopped.
The conventional electrochemical method for production of chlorine dioxide is performed either batchwise or continuously. In the batchwise process, a chlorite solution in an anode compartment in the diaphragm electrolytic cell is electrolyzed until the chlorite content becomes small, and thereafter, the anolyte is replaced by a fresh supply for starting electrolysis again. In the continuous process, a chlorite solution is continuously supplied to the anode compartment to maintain the chlorite content constant throughout the electrolysis. One defect of the batchwise process is that the operation is suspended at every anode replacement which is usually done every 1 to 3 days. To extend the replacement interval, much anolyte is necessary, resulting in an increase in equipment cost and installation area. In addition, the spent anolyte contains a considerable amount of chlorite and chlorine dioxide dissolved therein, and this means low efficiency in use of chlorite. Since a large amount of spent anolyte is discharged at one time, its treatment is difficult and requires a lot of personnel expenses. The continuous process provides a uniform concentration of chlorine dioxide, but much chlorine dioxide is dissolved away in the anolyte continuously discharged, and such discharged catholyte is difficult to treat, and the efficiency in use of chlorite is low. A continuous method is known that is designed for increasing the efficiency in use of chlorite by supplying a concentrated solution of chlorite prepared by adding a chlorite crystal to the discharged anolyte. But even in this method, salts in the crystal and those produced during electrolysis gradually build up and anolyte replacement is unavoidable. What is more, it is difficult to supply a constant amount of the crystal. Therefore, there is a great demand for an electrolytic process for production of chlorine dioxide which provides continuously a uniform concentration of chlorine dioxide, can be carried out with a compact apparatus, can be operated easily, discharges a spent electrolyte that can be treated easily, and which achieves highly efficient use of chlorite and high current efficiency.